1. Field of the Invention
This invention relates to a cosmetic applicator offering excellent utility as well as superior absorbency and barrier property with respect to cosmetic materials.
The present invention relates to a cosmetic applicator made of a rubber latex foam that exhibits bubble fineness, flexibility and bulk density of specified ranges.
To be more specific, the present invention relates to a cosmetic applicator made of a foam that simultaneously satisfies the three requirements of a bubble fineness corresponding to 9 bubbles/mm2 or more, flexibility of 0.7 N/cm2 or less in initial Young's modulus indicated by 50% compressive stress, and bulk density of 0.14 g/cm3 or less, wherein such foam is produced by way of outputting a stock solution for rubber latex foam production to a specified thickness and then irradiating microwaves to quickly solidify and vulcanize the solution.
The present invention also relates to a production method of the aforementioned cosmetic applicator.
2. Description of the Related Art
The foaming method (Dunlop method) and refrigerated solidification method (Talalay method) have been known as representative methods for producing rubber foam from rubber latex. Since the Talalay method cannot provide a flexible, fine, uniform foam, the Dunlop method is commonly adopted. In the Dunlop method, normally sodium silicofluoride is added to a latex compound solution and the mixture is agitated at high speed while blowing in air to produce a stock solution for latex foam production. This stock solution is then output into a liner having mold release property to a specified thickness, and then let stand in raised or room temperature until the solution solidifies by gelling and acquires a shale-retaining property.
In other words, since rubber latex is stable when it has an alkaline property, sodium silicofluoride is added to destabilize the rubber latex property via decomposition of sodium silicofluoride, thereby gradually changing the rubber latex property to acidity. This acid rubber latex is then gelated and solidified. However, the condition of this solidification process through gelling (solidification condition) has been the cause of repeated trials and errors in order to find optimal rubber latex formulations and molding conditions that yield higher productivity and consistent quality of foamed products. Today, the bubbled stock solution for latex foam production is generally solidified in room or raised temperature within 80 to 210 seconds after the solution is output.
In the present invention, “solidification” refers to solidification through gelling.
In the Dunlop method, the bubbles formed by way of injecting sodium silicofluoride and blowing in air cause the surface tension of the liquid in which the bubbles are formed to change when gelling is started, and once the bubbles start collapsing the bubbles will grow in size. The gelling point is generally set to 29±2° C., with a wait time of 80 to 200 seconds. This time must be ensured in the production process. Only after this time elapses will solidification progress with the initiation of gelling.
Flexibility of a foam is obtained by lowering its density. However, since the bubble wall becomes thinner as the density decreases, a lower density will cause fine bubbles to collapse during gelling and allow the bubbles to grow larger in size. For this reason, it was difficult to obtain fine bubbles at low density.
Based on the conventional Dunlop method described above, it was therefore unfeasible to obtain a foam satisfying the three requirements of fine bubble structure, flexibility and low density—the structural properties of foam desired by cosmetic applicators.
As a way to provide a flexible, low-density bubble layer, Japanese Patent Application Laid-open No. 6-30816 proposes use of a rubber latex stock solution that solidifies quickly. This technology is to adjust the pH of the rubber latex compound solution to between 11.7 and 12.00 and agitate the solution as sodium silicofluoride and air are blown in to produce a quick-solidifying stock solution for latex foam production. However, although the resulting solution can be applied on the surface of a seat material, as a solution for foam production the formulation does not provide a uniformly molded product. This is because solidification progresses when the solution is poured into a mold, thereby preventing formation of a uniform foam.
On the other hand, methods that utilize microwaves for sponge are also known. Examples include the method to improve the smoothness of sponge rubber surface by heating the surface with microwaves (Japanese Patent Application Laid-open No. 57-113048), and the method to adjust the sponge size by applying microwaves and hot air to an unvulcanized material (Japanese Patent Application Laid-open No. 8-108434). However, these methods do not utilize microwaves for the purpose of solidification.
Japanese Patent Application Laid-open No. 57-91252 proposes a foam production method that uses microwaves. Under this method, a stock solution for latex foam production is output onto a conveyor, with the sides heated by a hot-air blower and the top heated by a far-infrared or high-frequency electromagnetic heater. This method has been reported to reduce the losses resulting from the need to remove soiled or deformed sections on sides. Although it eliminates the side trimming process, this method does not use microwave irradiation during solidification.
On the other hand, another method is known whereby a relatively thick foam sheet is sliced into thin sheets and then stacked and joined together with the molded surface membrane with crushed bubbles facing the top membrane, thereby producing a cosmetic applicator (Japanese Utility Model Application Laid-open No. 59-40021). However, this method does not improve the bubble structure, flexibility or bulk density of the foam itself.